Material for Fire Tube

[StressFreak]

Hi,

I'm designing a gas heater in which a salt bath is heated by 2, 650mm OD fire tubes. These fire tubes are conected to a burner at one end, which delivers a 2.5m long flame with 750mm in diam, 1.6E6 Kcal/h and which has 2000°C at its hotest point (inside feather). For protecting the fire tubes from the flame there is another inside tube (length= 3m) made of Incoloy or Inconel. I'm wondering why do they use this material??

The flame has 2.5% of O2 (excess air) and it's expected to have 1300°C in contact with the sorrounding tube (not = to the inner wall temp).

Why can't I use a low carbon steel for this??

PS: Design pressure= atmospheric.

Thanks,
Diego.

 

[metengr]

You want to avoid direct flame impingement on the carbon steel tube otherwise you will have local surface oxidation that will destroy the tubes.

 

[StressFreak]

I suppose there are some flames (outer feather) liking the inner tube's wall, since it's only 600mm ID. What I do know is that direct flame on Incoloy will melt it down. Not taking into account the cost and time that will demand to obtain this material.

The question is how can I estimate the service life of a low carbon steel tube exposed to those 1300 degC flames with 2.5% excess O2??

 

[GenB]

c/s starts deteriorating at 850 F prolonged use so there is your answer,
incoloy will easily takes 2000F or more
genb

 

[StressFreak]

I don't aggree with 850°F as a limit temperature for carbon steel. Take SA 516 Gr70 for example, it can be used at up to 1000°F according to asme div VIII.

I know Inconel is the best material for this application; but the question is: if I'd used CS, how long will it last? For this application a life expectancy of only 1 year would be fine. Besides, there are a lot of direct fire vessels made of CS and the last many years, take a cooking pan for example...

Diego.

 

[PAN]

StressFreak,

Creep strength should be considered for carbon steel at temperature 1300 C.

Frankly speaking, I do not have experience to design fire tube of gas heater with 1 year design life.

 

[MJCronin]

Stressfreak,

You are incorrect,..... ASME VIII does NOT permit nor encourage use of carbon steel in continuous duty over 800F. I do not understand how you came to this conclusion.....

It has long been known that carbon steel will degrade via a mechanism known as "graphitization" above 800F.

See:

http://www.nationalboard.org/NationalBoard/Articles/Classics/classic24.aspx

-MJC

 

[StressFreak]

MJCronin,

See ASME Sec II Part D, Table 1A. Now, if you say this is not applicable, tell me where in the ASME can I find what you say is the limit.

Apart from this, I know for a fact that fire tubes are made of plain CS (SA 106 GrB) and they work at above 500°C (932°F). They last many years and limiting factor is the salt corrosion (about 0.1mm per year).

Diego.

 

[EdStainless]

There are three real limits to using CS in direct fired application.
1. Do they have enough strength at the use temp?
2. What will the external corrosion rate be in the hotter sections?
3. What will the internal corrosion (oxidation) rate be in the hotter sections.

Usually the answers to all of these are bad news. These thin liners result in huge gains of durability.

I have seen direct flame-exposed test stands fail in weeks, where the Ni alloy version ran for many years.
Now if you only need a year could you use 430 stainless? It might be worth a look. The next step might be 310, and after that 800. There are options other than 600.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[StressFreak]

EdStainless,

In this case the CS will have practically no stress since it's only a liner that separates the fire tube (SA 106 Gr.B) from the flame. The fluid outside this liner is still combustion gas and on the inside there is a flame.
The flame may be touching the liner's inside wall; but based on calculations we don't expect a temp higher than 1000°C (1832°F). What I don't know is what will the corrosion rate be, so my worry is whether this liner will last a week or 1 year (accepted design life).

May be I should insist on Inconel and forget about it...

thanks,
Diego.

 

[EdStainless]

Is the unit going to be continously fired? I mean all of the time at a constant rate? If not the CS will form oxide and it will spall off. I really don't think that there is any chance for CS to last for even months.
Some of the in between alloys might though....

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm